In the text which follows, “trailer” will be understood to be any type of trailer such as, for example, a drawbar trailer or even semitrailer or, in the case of a plurality of trailers also combinations of trailers of the same type or of different types.
Usually, when the friction brake system of the traction vehicle is activated, the friction brake system of the trailer is also activated or actuated or applied. In the case of electropneumatic brake systems or electronically controlled brake systems (EBS) in the traction vehicle, in this context a braking request is generated, for example, by the driver or by a driver assistance system (ESP, ACC) as electric, and in parallel also as a pneumatic, braking request signal, wherein in the case of trailer brake systems without traction controller (ABS) at least the pneumatic braking request signal is applied to the friction brake system of the trailer, in order to generate a brake pressure in the friction brake system of the trailer as a function of the braking request. For this purpose, the electrical and also the pneumatic braking request signals are applied to a traction-vehicle-side trailer brake module of the electronically controlled brake system (EBS), which trailer brake module then modulates a pneumatic brake signal to a “brake” coupling head for the trailer. The electronically controlled brake system (EBS) usually contains ABS routines and carries them out within the scope of a brake-slip-controlled braking operation. In this context, the traction vehicle and the trailer are decelerated to a more or less equal degree even in the case of a brake-slip-controlled braking operation.
The coordination of the braking between the traction vehicle and the trailer is regulated in the standard ECE R 13. The latter contains what are referred to as compatibility bands which define a ratio between the braking “z” and the braking force of the trailer or the pressure at the “brake” coupling head of the traction vehicle in pneumatic brake systems.
Last but not least, driver assistance systems, for example in the form of movement dynamics control systems such as ESP (electronic stability program) or RSP (roll stability program) in which motion of the vehicle is monitored by a sensor system (acceleration sensors, yaw rate sensors) and the traction vehicle-trailer combination is, if appropriate, braked automatically, are known for electrical, electropneumatic and electrohydraulic brake systems. Furthermore, such driver assistance systems can, for example, also contain an ACC (Adaptive Cruise Control) system in which the distance from a vehicle travelling ahead or cutting in is adjusted automatically to a setpoint distance. Such driver assistance systems have in common the fact that they activate the brake system of the traction vehicle and also of the trailer automatically or else activate only the brakes of individual wheels of the traction vehicle and of the trailer when specific criteria are met, for example undershooting of the setpoint distance in the case of an ACC system or detection of an unstable driving state in the case of ESP.
In the case of brake devices of traction vehicle-trailer combinations in which the brake system for the traction vehicle is equipped with a traction controller, but the brake system of the trailer is not, the problem arises that in the case of a braking process, in particular in bends, the trailer can experience a loss of lateral guidance, which, in turn, causes the trailer to swing out with respect to the traction vehicle. Such swinging out of the trailer both in the case of an automatic braking intervention by a driver assistance system as well as in the case of a braking operation which is triggered by the driver himself can occur.
However, this problem also occurs in trailers whose brake system has a traction controller for the axles which are present, but in which brake system the brake slip is determined by wheel speed sensors on fewer axles than the number of axles which are present, for example on only the wheels of one axle. In other words, wheel speed sensors are present only on the wheels of one axle or of a number of the axles, in order to determine the wheel brake slip at the one axle or at some of the axles of the axles which are present. Such a traction controller is rather imprecise, in particular if, for example, the single axle of the trailer at which the determination of the wheel brake slip is carried out has the highest load, and consequently the other axles which are loaded to a lesser degree then experience blocking by the traction control process which is carried out at the one axle after wheel slip has been detected. In this case, insufficient lateral guiding force to avoid swinging out of the trailer may also be generated at the wheels of the axles of the trailer.
In order to remedy this problem, EP 0 433 858 A2, which is of the generic type, proposes detecting swinging out of a semitrailer with respect to the traction vehicle by a rotational angle sensor which is arranged on the swivel joint between the semitrailer and the traction vehicle, and, in the event of a permissible rotational angle being exceeded, to release the trailer brake in order to increase the lateral guidance forces of the wheels of the trailer. However, such procedure requires additional hardware and is therefore relatively costly.